Many people use diet drinks as mixers with alcohol because they want avoid the calories associated with sugary beverages. But a new study suggests there is a unintended consequence of opting for a diet mixer – greater intoxication.

“I think no one has any idea that this is the case,” said Cecile Marczinski, the lead researcher and professor of psychology at Northern Kentucky University.

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Even the researchers were surprised that substituting a sugary beverage with a diet drink boosts the blood-alcohol level by such a big extent – 18 per cent.

In has long been known that consuming food will slow the passage of alcohol from the stomach into the lower intestine where it is absorbed into the bloodstream. In a similar fashion, sugar in the form of a drink will act like a food and slow the absorption of alcohol.

But with a low-calorie diet beverage, there is little to hinder the movement of alcohol through the gastrointestinal tract, leading to a relatively quick spike in blood alcohol levels.

For the study, the researchers recruited 16 volunteers – eight males and eight females. Each participant came into the research centre on three separate days. On one occasion, they were given vodka with a diet beverage. (The number of drinks they received – totalling either three or four ounces of alcohol – depended on their body weight.) On another occasion, they got vodka with a surgery drink. And one time they were served a non-alcoholic placebo.

“The drinks – all three to four of them – were consumed within 10 minutes. This methodology allows the blood alcohol to peak and decline quickly so that the subjects can leave the laboratory in about five hours,” Marczinski said.

The results revealed that the vodka and the regular mix produced an average blood-alcohol concentration of .077 per cent among the study participants. When the diet beverage was used, their average blood-alcohol level shot up to .091. That represents an increase of 18 per cent. But more important, it pushed them over .08 – the legal limit for driving.

“When you consume alcohol with a diet drink, then just be aware of the fact this is going to increase your blood-alcohol concentration and you need to be careful,” Marczinski said.

This isn’t the first time that a study has hinted that diet beverages could pose an intoxication hazard. For almost a decade, Dennis Thombs at the University of North Texas Health Science Center has been conducting random tests of patrons leaving bars and nightclubs. His tests have often found that people who consumed alcohol with a diet drink tended to have significantly higher alcohol levels than those using a regular mix.

Marczinski’s new study, which will be published in the journal Alcoholism: Clinical & Experimental Research, went a step further by doing the measurements in a lab, where the effects of different mixes could be rigorously compared and quantified.

While conducting his research, Thombs also noticed that females tend be the ones who specifically order diet mixes.

“I am assuming that women are somewhat more weight conscious than men, so they are trying to reduce their caloric intake when they are out at a bar or nightclub drinking,” Thombs said. “But they are probably unaware that this is creating a situation where they are going to become perhaps more intoxicated than they intended.”

Marczinski agrees with Thombs’ assessment. “Women, especially, are really conscious of calories,” she said. “But it is far more harmful to your brain and your liver to have a higher blood alcohol concentration as opposed to a couple of extra calories in a drink.”

Thombs believes the public should be made aware of the potential risk of mixing alcohol with diet beverages. But he is also concerned that a public education campaign could be counterproductive. “Some people could use this information not to protect themselves, but to get themselves more intoxicated.”